Background: Although radiotherapy is one of the most effective strategies in the treatment of cancers, it isassociated with short and long term side effects on normal tissues. Zataria multiflora Boiss (Laminacea) (ZM)has several biological properties such as antioxidant and anti-inflammation activities.Here we investigated cellkilling effects of a hydroalcoholic Zataria multiflora extract on cell death induced by ionizing radiation in a humanglioblastoma cell line (A172) and human non-malignant fibroblasts (HFFF2) in vitro. Materials and Methods: A172and HFFF2 cells were treated with a hydroalcoholic extract of dried aerial parts of Zataria multiflora at differentconcentrations (25, 50, 100, 150 and 200 μg/ml) and then exposed to ionizing radiation (IR). Cell proliferationand DNA fragmentation were evaluated. Thymol content in the extract was analyzed and quantified by HPLCmethods. Results: A172 cell proliferation was significantly inhibited by ZM. The percentage cell survival was91.8±8.57 for cells treated with 200 μg/ml of ZM extract alone while it was 76.0±4.27 and 66.2±8.42 for cellstreated with ZM and exposed to IR at doses of 3Gy and 6Gy, respectively. Radiation-induced apoptosis in A172cells was significantly increased following treatment with ZM at doses of 200 μg/ml. ZM extract did not exhibitany enhanced cell killing effects and apoptosis caused by IR on HFFF2 cells. Conclusions: These data showselective radiosensitization effects of ZM in A172 cells apparently due to increased radiation-induced apoptosis.